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Stopping distance and matching speed to the road surface

General Knowledge4 minManual section 2.6.1–2.6.2

Driving too fast is a major cause of fatal crashes. You must adjust speed for traction, curves, visibility, traffic and hills. This lesson covers the stopping-distance formula and its numbers, how speed and weight change it, and slippery roads, black ice and hydroplaning.

The stopping distance formula (2.6.1)

Perception distance + Reaction distance + Braking distance = Total stopping distance. The numbers below are for 55 mph in ideal conditions.

  • Perception distance: how far you travel from the moment your eyes see a hazard until your brain recognizes it. An alert driver needs about 1¾ seconds — about 142 feet at 55 mph. Mental and physical conditions, visibility and the hazard itself can change it a lot.
  • Reaction distance: how far you travel from recognizing the hazard until your foot actually hits the brake. The average reaction time is ¾ second to 1 second — about 61 feet at 55 mph.
  • Braking distance: how far you travel while braking. At 55 mph on dry pavement with good brakes, about 216 feet.
  • Total: at 55 mph you travel at least 419 feet before stopping (142 + 61 + 216). That is longer than a football field.

How speed and weight change stopping

The faster you go, the greater the impact (striking power) and the longer the braking distance — and they grow with the square of speed:

  • Double the speed (20 → 40 mph): impact and braking distance are 4 times greater.
  • Triple the speed (20 → 60 mph): 9 times greater. At 60 mph, stopping distance is longer than a football field.
  • Four times the speed (20 → 80 mph): 16 times greater.

Slowing down is the way to cut braking distance.

Weight: a heavier vehicle makes the brakes work harder and absorb more heat. But brakes, tires, springs and shock absorbers on heavy vehicles are designed to work best fully loaded. Empty trucks need longer stopping distances because an empty vehicle has less traction. So “empty trucks brake best” is false.

Traction and slippery surfaces (2.6.2)

You cannot steer or brake without traction — the friction between the tires and the road. On slippery roads it takes longer to stop and it is harder to turn without skidding.

  • Wet road: stopping distance can double. Reduce speed by about one-third (for example, from 55 to about 35 mph).
  • Packed snow: reduce speed by half or more.
  • Ice: slow to a crawl and stop driving as soon as you can do so safely.

Signs of a slippery road

  • Shaded areas stay icy long after open areas have melted.
  • Bridges freeze before the road does. Be extra careful near 32°F.
  • Melting ice: slight melting makes ice wet, and wet ice is much more slippery than dry ice.
  • Black ice: a thin layer so clear you can see the road through it; it makes the road look wet. Whenever it is below freezing and the road looks wet, watch for black ice.
  • Vehicle icing: roll down the window and touch the front of the mirror, its support or the antenna. Ice forming there means the road surface is probably icing too.
  • Just after rain begins: water mixes with oil on the road and makes it very slippery. If the rain continues, it washes the oil away.

Hydroplaning

When water or slush collects on the road, your tires can ride on top of it like water skis and lose contact with the road — this is hydroplaning. You may not be able to steer or brake.

  • To regain control, release the accelerator and push in the clutch. This slows the vehicle and lets the wheels turn freely.
  • Do not use the brakes to slow down while hydroplaning.
  • If the drive wheels start to skid, push in the clutch so they can turn freely.

It does not take much water. With plenty of water on the road, you can hydroplane at just 30 mph. It is more likely with low tire pressure or worn tread (shallow grooves cannot carry water away).

Watch for signs of standing water: clear reflections, tire splashes and raindrops on the road.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 2.6.1–2.6.2

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

Preparing your questions…

Review questions with answers

  1. Total stopping distance is the sum of which three distances?
    1. Perception distance, reaction distance and braking distance
    2. Following distance, braking distance and skid distance
    3. Reaction time, speed and weight
    4. Sight distance, reaction distance and coasting distance
    Show answer
    Perception distance, reaction distance and braking distance

    Perception + reaction + braking = total stopping distance.

  2. At 55 mph in ideal conditions, a vehicle travels a minimum of about how far before stopping?
    1. 216 feet
    2. 300 feet
    3. 419 feet
    4. 600 feet
    Show answer
    419 feet

    142 + 61 + 216 = about 419 feet total.

  3. Perception distance is the distance traveled from the time you see a hazard until:
    1. Your foot hits the brake
    2. Your brain recognizes it
    3. The vehicle stops
    4. The brakes begin to heat up
    Show answer
    Your brain recognizes it

    Perception distance ends when your brain recognizes the hazard.

  4. The average perception time for an alert driver is about:
    1. ¼ second
    2. ½ second
    3. 4 seconds
    4. 1¾ seconds
    Show answer
    1¾ seconds

    About 1¾ seconds, which is about 142 feet at 55 mph.

  5. The average driver's reaction time is about:
    1. ¾ second to 1 second
    2. 2 to 3 seconds
    3. 4 to 5 seconds
    4. Less than ¼ second
    Show answer
    ¾ second to 1 second

    Reaction time is ¾ to 1 second, about 61 feet at 55 mph.

  6. At 55 mph on dry pavement with good brakes, braking distance is about:
    1. 61 feet
    2. 142 feet
    3. 216 feet
    4. 419 feet
    Show answer
    216 feet

    Braking distance alone at 55 mph is about 216 feet.

  7. If you double your speed from 20 to 40 mph, your braking distance becomes:
    1. 2 times longer
    2. 4 times longer
    3. The same
    4. 3 times longer
    Show answer
    4 times longer

    Braking distance grows with the square of speed: double speed = 4 times.

  8. Going from 20 mph to 60 mph makes impact and braking distance about how many times greater?
    1. 3 times
    2. 6 times
    3. 16 times
    4. 9 times
    Show answer
    9 times

    Triple the speed gives 9 times the impact and braking distance.

  9. Compared to 20 mph, at 80 mph the impact and braking distance are about:
    1. 16 times greater
    2. 4 times greater
    3. 8 times greater
    4. 10 times greater
    Show answer
    16 times greater

    Four times the speed gives 16 times the impact and braking distance.

  10. An empty truck can stop in a shorter distance than a loaded one.
    1. True
    2. False
    Show answer
    False

    False. Empty trucks need greater stopping distances because they have less traction.

  11. On a wet road, how much should you reduce your speed?
    1. Not at all if your tires are new
    2. By about 10 percent
    3. By about one-third
    4. By at least two-thirds
    Show answer
    By about one-third

    Reduce speed by about one-third on a wet road (for example, 55 to about 35 mph).

  12. On packed snow, you should reduce your speed by:
    1. Half or more
    2. About one-quarter
    3. About 5 mph
    4. Nothing if you have chains
    Show answer
    Half or more

    On packed snow, slow by half or more; on ice, slow to a crawl.

  13. What is black ice?
    1. Ice mixed with dirt and oil
    2. Thick ice on shaded mountain roads
    3. Ice that forms only on bridges
    4. A thin, clear layer of ice that makes the road look wet
    Show answer
    A thin, clear layer of ice that makes the road look wet

    Black ice is so clear you can see the road through it; watch for it when below freezing and the road looks wet.

  14. Which of these is usually the most slippery?
    1. Dry, cold ice
    2. Ice that is slightly melting and wet
    3. Packed snow
    4. Wet pavement
    Show answer
    Ice that is slightly melting and wet

    Slight melting makes ice wet, and wet ice is much more slippery.

  15. When does a road become especially slippery from rain?
    1. After it has rained for several hours
    2. Only at night
    3. Just after the rain begins
    4. Only when the temperature is above 80°F
    Show answer
    Just after the rain begins

    Early rain mixes with oil on the road; continued rain washes the oil away.

  16. Your vehicle starts to hydroplane. What should you do?
    1. Release the accelerator and push in the clutch
    2. Brake hard to slow down
    3. Steer sharply toward the shoulder
    4. Speed up to get through the water
    Show answer
    Release the accelerator and push in the clutch

    Releasing the accelerator and pushing the clutch slows the vehicle and lets the wheels turn freely; do not brake.

  17. Hydroplaning can happen at speeds as low as:
    1. 10 mph
    2. 55 mph
    3. 45 mph
    4. 30 mph
    Show answer
    30 mph

    With a lot of water, hydroplaning can occur as low as 30 mph.

  18. Which condition makes hydroplaning MORE likely?
    1. High tire pressure
    2. Low tire pressure or worn tread
    3. A heavy load
    4. Driving below 20 mph
    Show answer
    Low tire pressure or worn tread

    Shallow tread grooves cannot carry water away, and low pressure makes it worse.

  19. An easy way to check for ice while driving is to:
    1. Tap the brakes hard
    2. Watch the temperature gauge for the engine
    3. Reach out the window and touch the front of the mirror, its support or the antenna
    4. Listen to the tires
    Show answer
    Reach out the window and touch the front of the mirror, its support or the antenna

    If ice is forming on the mirror or antenna, the road is probably starting to ice.

  20. When the temperature drops near 32°F, which part of the road freezes first?
    1. Bridges
    2. Uphill grades
    3. Sunny open stretches
    4. Freshly paved lanes
    Show answer
    Bridges

    Bridges freeze before the road does.

  21. Which is a sign of standing water that could cause hydroplaning?
    1. Dry dust blowing
    2. A rumble strip
    3. Fresh white lines
    4. Clear reflections, tire splashes and raindrops on the road
    Show answer
    Clear reflections, tire splashes and raindrops on the road

    Reflections, splashes and raindrops on the road show water is collecting.

  22. Shaded parts of the road melt at the same time as open areas.
    1. True
    2. False
    Show answer
    False

    False. Shaded areas stay icy long after open areas have melted.

  23. Compared with a dry road, a wet road can make your stopping distance:
    1. Shorter
    2. About the same
    3. Ten times longer
    4. Double
    Show answer
    Double

    Wet roads can double stopping distance, so slow down by about one-third.

  24. What is traction?
    1. The friction between the tires and the road
    2. The pulling power of the engine
    3. The weight on the steering axle
    4. The space between dual tires
    Show answer
    The friction between the tires and the road

    Traction is tire-to-road friction. Without it you cannot steer or brake.